CURA-1923: SliceInfo: Sending data threaded
CURA-1923: SliceInfo: Sending data threaded

The idea is to make the sending process, so it won't block the UI until the data is sent or the timeout has reached.
This commit does the following:
* Increasing the timeout to 5s
-> As we are sending our data threaded now this won't hurt. At least for clients in slow networks.
* Moving section for "sending data to info_url" into a seperate thread.
* Starting the thread instead and collecting all threads in a list
* Removing all threads from the list, if they have already finished their work.
* When the SliceInfo Extension gets __del__'d, it will wait for the threads until they finish.

Contributes to CURA-1923

hmflash/Cura,totalretribution/Cura,fieldOfView/Cura,hmflash/Cura,totalretribution/Cura,senttech/Cura,senttech/Cura,Curahelper/Cura,ynotstartups/Wanhao,fieldOfView/Cura,Curahelper/Cura,ynotstartups/Wanhao